The present invention relates to the treatment of cardiac arrhythmias such as atrial fibrillation.
The normal contractions of the heart muscle arrives from electrical impulses generated at a focus within the heart and transmitted through the heart muscle tissue or xe2x80x9cmyocardialxe2x80x9d tissue. This tissue includes fibers which can carry electrical signals. The tissue of the pulmonary vein normally merges with the myocardial tissue of the heart wall at a border near the opening or ostium of the pulmonary vein. In some individuals, fibers of myocardial tissue extend from the wall of the left atrium, through the ostium and along the wall of the pulmonary vein. In some individuals, elongated strands of myocardial tissue extend within the wall of pulmonary vein away from the heart (distally), so that the strains of myocardial tissue project beyond the normal border.
It has been recognized that atrial fibrillation can be caused by an abnormal electrical focus in such strands of myocardial tissue within the pulmonary vein. Because these strains of myocardial tissue merge with myocardial tissue of the heart wall, electrical signals from such an abnormal focus can propagate proximately along one or more strands of myocardial tissue through the myocardial tissue and into the heart wall itself, resulting in abnormal contractions.
It has been recognized that this condition can be treated by locating the abnormal focus of an electrical signal and ablating (i.e., killing or damaging), the tissue at the focus so that the tissue at the focus is replaced by electrically inert scar tissue. However, the focus normally can be found only by a process of mapping the electrophysiological potentials within the heart and in the myocardial fibers of the pulmonary vein. There are significant practical difficulties in mapping the electrical potentials. Moreover, the abnormal potentials which cause atrial fibrillation often are intermittent. Thus, the physician must attempt to map the abnormal potentials while the patient is experiencing an episode of atrial fibrillation.
Another approach that has been employed is to ablate the tissue of the heart wall, so as to form a continuous loop of electrically inert scar tissue extending entirely around the region of the heart wall which contains the ostium of the pulmonary veins. In this approach, the scar tissue prevents the abnormal electrical impulses from propagating into the remainder of the atrial wall. In a variant of this approach, a similar loop-like scar can be formed around the ostium of a single pulmonary vein or in the wall of the pulmonary vein itself proximal to the focus so as to block propagation of the abnormal electrical impulses. Such scar tissue can be created by forming a surgical incision; by applying energies such as radio frequency energy, electrical energy, heat, cold, intense light such as laser light; or ultrasonic energy. Chemical ablation agents also can be employed. Techniques which seek to form a loop like lesion to form a complete conduction block between the focus and the major portion of the myocardial tissue are referred to herein as xe2x80x9cloop blocking techniques.xe2x80x9d
Loop blocking techniques are advantageous because they do not require electrophysiological mapping sufficient to locate the exact focus. However, if a complete loop is not formed, the procedure can fail. Moreover, ablating complete, closed loops without appreciable gaps presents certain difficulties. Thus, some attempts to form a complete loop of ablated tissue around the entire circumference of the pulmonary vein have left significant unablated regions and thus have not formed a complete conduction block. Other attempts have resulted in burning or scaring of adjacent tissues such as nerves. Moreover, attempts to form the required scar tissue using some types of ablation instruments such as radio frequency ablation and unfocused ultrasonic ablation have caused thromboses or stenosis of the pulmonary vein. The potential for these undesirable side effects varies directly with the amount of tissue ablated. In addition, the amount of energy which must be applied in an ablation procedure varies directly with the amount of tissue ablated, an the more energy that need to be applied, the larger the ablation element typically needs to be. Particularly where an ablation element must be introduced into the heart through a catheter, the size of the ablation element and hence the energy delivery capacity per unit time of the ablation element is limited. While these difficulties can be alleviated or eliminated by the use of focused ultrasonic ablation as taught, for example, in copending, commonly assigned U.S. Provisional Patent Application No. 60/218,641 filed Jul. 13, 2000, now U.S. patent application Ser. No. 09/905,227 xe2x80x9cThermal Treatment Methods and Apparatus With Focused Energy Applicationxe2x80x9d; Ser. No. 09/904,963 xe2x80x9cEnergy Application With Inflatable Annular Lensxe2x80x9d; and Ser. No. 09/904,620 xe2x80x9cUltrasonic Transducers,xe2x80x9d the disclosure of which are incorporated by reference herein, further alternatives would be desirable.
One aspect of the present invention provides methods of treating cardiac arrhythmias such as atrial fibrillation which arise from abnormal electrical impulses conducted along strands of myocardial tissue in the wall of a blood vessel. A method according to this aspect of the invention desirably includes the steps of finding the locations of myocardial strands around the circumference of the blood vessel and ablating these strands proximal to the focus. Most preferably, the strands are ablated without ablating a continuous loop. Apparatus are also disclosed that may be used to perform this method.